Why not put the complete 1984 motor in the 1979 frame?Â
Matching numbers. I want to keep the registration process simple for and keep the bike matching for the next owner.
Firstly, let me say very clearly that the whole "matching numbers" business is complete and utter tripe. BMW frequently put bikes out the door with non-matching numbers - the result of (for example) a frame being defected, or an engine not coming up to spec. If you have matching numbers on a bike - Yay, but it means absolutely nothing.
This sort of nonsense is perpetuated by the same mob who a few years ago got hold of production records and discovered that certain gearbox number sequences were manufactured by BMW and some by specialist gearbox manufacturer Getrag. The drivel then peddled was that "genuine" BMW gearboxes are better than Getrag manufactured ones - a complete nonsense and akin to saying "my Romanian (guest workers) are better than your Romanians.
Ok, now I have that off my chest, if you really, really must do it, all of your 1984 internals will fit in 1979 cases.
BUT, I'd be very sure that you have the tools, knowledge and skill to do this, else the outcome is likely to be two sets of engine cases (and their innards) open to the elements and decaying.
The cylinder heads on the 1984 have bigger valves than the 1979 so they will be "non genuine", likewise the 1984 barrels will have a Nicasil liner whereas "genuine" 1979 ones are iron bore. The 1984 pistons have a slightly higher deck height above the top ring to increase compression - non "genuine" again.
The 1979 beancan will house points whereas the 1984 one will have a hall effect trigger. I am old and cynical now and am no longer in love with the wonderous concept of electronic ignition as I once was - I would use the points. If you don't want to use points, your cheapest alternative would be a Boyer Bransden ignition system and the points beancan. To use the later Hall effect you will need to buy the connecting harness ($AU65 - I've bought one in the recent past) an ignition control unit and a more modern coil (and given that this is to "genuine" you are looking at around $AU200 for the "genuine" coil for the hall effect ignition, Dyna coils are about half the price and have a better reputation for longevity, but the original 6 volt in series coils will still be working when Christ comes again - they are also "genuine" for the 1979.
All the engine bits will happily fit in the 1979 cases.
The other differences to be aware of is that the 1979 engine has the older "heavy" flywheel, whereas the 1984 uses the skeleton "clutch carrier". As the gearbox has a different protrusion distance to match either the flywheel or the clutch carrier - if you are using a 1979 gearbox you will need the flywheel, if using a 1981 or later gearbox you need the light weight clutch carrier.
If it were me i would simply put the 1984 engine (with 79 flywheel if needed) into the bike and then liberally lubricate the 79 engine internals, throw in some long term inhibitor and then plastic wrap it. Then if one day in the future a 1979 R65 is worth more than a 1984 R65 and the then owner wants to unlock that extra cash - then they can do the work then.
